This invention relates to a flowmeter and control method thereof to measure flow rate of fluid by using a pulse signal having a frequency corresponding to a flow rate from a sensor, a flow detecting switch and control method thereof to compare a measured flow rate and a preset flow rate value and to output a digital output corresponding to a comparison result toward an external apparatus, and a recording medium having recorded a control program.
And, the present invention relates also to a flowmeter to measure an injection rate of each fluid so that a plurality of fluids to be measured can be fixed in a fixed percentage in one tank or to measure an injection rate of each fluid so that a plurality of fluids with different temperature or the like can be mixed in one tank, a control method thereof, and a recording medium having recorded a control program.
Some kinds of liquids are often transferred, mixed and saved in a liquid tank in a fixed percentage in a manufacturing process using liquid medicine and in a cleaning process after the manufacturing process in a semiconductor manufacturing facility, a liquid crystal manufacturing facility, and the like.
An outline of such a conventional liquid mixing system is shown in FIG. 42.
In FIG. 42, the liquid mixing system 100 has reservoirs 101A-101C to reserve respective liquids to be mixed, a liquid tank 102 to mix the liquids, solenoid valves 103A-103C provided on respective flow-channels between the reservoirs 101A-101C and the liquid tank 102 in order to carry out injection/injection stop of the respective liquids, a plurality of flowmeters 104A-104C provided on respective flow-channels between the solenoid valves 103A-103C and the liquid tank 102 in order to output flow detection signals FA-FC, for example analog signals, having current values of 4-20 [mA] corresponding to respective flow rate values every fixed measuring period, a plurality of totalizers 105A-105C to output on/off control signals SC for opening and closing the respective solenoid valves 103A-103C when fixed integrated batch flow rates are obtained, and a concentrated monitor 106 to receive the batch output signals BA-BC and to output reset signals SR to reset the integrated batch flow rates of the totalizers 105A-105C. The plurality of totalizers 105A-105C also integrate the respective flow rates on the basis of the flow detection signals FA-FC and output respective batch output signals BA-BC to inform of the completion of the integrating when the respective fixed integrated batch flow rates are obtained. Here, in this specification, the words related to xe2x80x9cintegratexe2x80x9d mean almost the same as of xe2x80x9ctotalizexe2x80x9d. A pump M in FIG. 42 is controlled by a sequencer 106A.
The concentrated monitor 106 has the sequencer 106A to carry out the control of the whole liquid mixing system 100 and a display touch panel 106B to carry out the input of various data and the indication of data.
And, the above totalizers 105A-105C are set on a local monitoring board 107 provided at the flowmeters 104A-104C side (i.e. local side).
Summary of operation of this liquid mixing system 100 is described hereinafter. Here, only a liquid system of the reservoir 101A will be described because operation of each liquid system is the same. And, it is assumed that a liquid injection rate (i.e. an integrated batch flow rate) has been preset to the totalizer 105A in advance.
When the sequencer 106A of the concentrated monitor outputs the reset signal SR, the integrated batch flow rate of the totalizer 105A is reset, and this totalizer 105A outputs on/off control signal SC to the solenoid valve 103A.
By this, ON signal the solenoid valve 103A comes to an opened state, and the liquid reserved in the reservoir 101A is injected in the liquid tank 102.
With this, the flowmeter 104A outputs the flow detection signal FA corresponding to the flow rate to the totalizer 105A.
When the integrated batch flow rate reaches the preset integrated batch flow rate, the totalizer 105A outputs the on/off control signal SC to the solenoid valve 103A, and the solenoid valve 103A comes to a closed state with this off signal. And, the injection of the liquid to the liquid tank 102 stops, and the batch output signal BA is outputted to the sequencer 106A of the concentrated monitor 106.
As a result, the sequencer 106A indicates the liquid injection having closed on the display touch panel 106B.
By the way, a frequency meter, a rotation speedometer, a pulse counter and the like are known as devices to measure a signal from a sensor which outputs a pulse signal based on measurement of a physical quantity.
For example, a capacitance type pressure sensor forms an oscillator and generates a signal with a frequency corresponding to the pressure. Then, pressure can be measured by measuring the frequency of the signal from this pressure sensor. And, in a vane wheel type flow sensor a magnetized vane wheel is rotated by a flow, and a secured Hall element detects the magnetism thereby to output a pulse signal having a frequency corresponding to the flow rate. The flow rate can be measured by measuring the frequency of this pulse signal.
There is a Karman vortex type flow sensor as a flow sensor of the above flowmeter or as one of such a physical quantity measuring apparatus. The Karman vortex type flow sensor detects the frequency of Karman vortexes by means of a supersonic wave sensor or a pressure-electricity element sensor in order to measure the flow rate of the fluid such as gas or liquid.
The flow measurement range and the frequency of Karman vortexes corresponding to the above flow rate are decided by a diameter of a flow-channel and material of the pipe portion of the flow sensor.
Here, an instantaneous flow rate value Q is obtained from the next linear expression (1) by using the frequency value f.
Q=axc3x97f+bxe2x80x83xe2x80x83(1)
And, because the expression (1) does not give an accurate result, the following expression (2) is applied, wherein the flow measurement range is divided into a plurality of sections n, and a linear expression for each section i (i=1xe2x88x92n) is defined thereby to secure nearly a full scale accuracy xc2x11%. (Japanese Patent Application Laid-open NO. 60-238720)
xe2x80x83Q=aixc3x97f+bixe2x80x83xe2x80x83(2)
For further improving the accuracy, the following expression (3) is proposed thereby to secure a full scale accuracy xc2x10.5%, wherein the above ai value and bi value are decided for each device and the individually decided values, i.e. corrected coefficients, Ai(s) and Bi(s) (i=1xe2x88x92n) are stored in a RAM or an EEPROM. (Japanese Patent Application NO. 9-345742)
Q=Aixc3x97f+Bixe2x80x83xe2x80x83(3)
And, as for the above frequency value f, the measuring accuracy should be studied. Generally, when the frequency of a pulse signal is measured, the pulse number inputted during a fixed time-period (a gate-time) is counted, or the frequency is calculated from the reciprocal number of the measured period of the input pulse. However, in the flow measurement, the frequency of the generated pulse is high (the period is short) in a large flow and is low (the period is long) in a small flow. Accordingly, a sufficient measuring accuracy has to be obtained in each of the large and small flows.
Therefore, a frequency measuring apparatus having a counter controlling means to control a count operation with a timer, a clock generator, a clock counter, and a clock and a processing means to detect and count a pulse signal and to carry out a fixed arithmetic control is proposed. (Japanese Patent Application Laid-open NO. 5-297036)
According to this frequency measuring apparatus, a time not less than 2 times of the period of a measurement bottom frequency is set to the above timer, and the detection and the count of the pulse signal is started. And, when a pulse signal is detected, the above counter controlling means is controlled so that the above clock counter starts the count of a clock from the above clock generator, and the count value of the clock counter is checked every time the pulse signal is detected. And, the detection and the count continue when the above count value does not reach a specified value. Meanwhile, when the above count value has reached the specified value, the above detection and the count are stopped, and a measured frequency value is calculated from the count value of the above clock counter by controlling the above counter controlling means. And, when the count of the clock has not completed at the time of the completion of the clocking of the timer, the detection and the count of the pulse signal is stopped, and the above counter controlling means is controlled to close the count by the clock counter.
With the above, N-time of the period of the pulse signal is counted by the clock counter, and a required measurement resolvability can be obtained regardless of the frequency value of the pulse signal by continuing the integration (i.e. totalizing) of the period until the count value secures a desirable resolvability.
And, an average period calculation apparatus of the output pulse of a Karman vortex type airflow rate sensor is also proposed for the purpose of enhancing a calculation accuracy of an average period in a large flow and improving responsibility in a small flow. (Japanese Patent Application Laid-open NO. 5-70086)
This average period calculation apparatus has a pulse number counting means, a pulse period measuring means, a pulse period comparing means to compare a measuring period and a first fixed time, and an average period calculating means. Specifically, the average period calculating means calculates the average of the period of a pulse signal, when the pulse number corresponding to a first specified value is counted by the above pulse number counting means from the time of the completion of the last average calculation, when a comparison result of xe2x80x9clargerxe2x80x9d are outputted from the above pulse period comparing means by the number of times corresponded to a second specified value, or when a state that a comparison result of xe2x80x9clargerxe2x80x9d subsequently to a comparison result of xe2x80x9csmallerxe2x80x9d has occurred in the pulse period comparison means by the number of times corresponding to a third specified value. The average period calculation apparatus further has a second average period calculating means to calculate an average period from a comparison result by the above pulse period comparing means using the second specified value, or a second pulse period comparing means to compare the above measuring period and the second fixed time and a second average period calculating means to calculate an average period from the output of the above second pulse period comparing means.
With these structures an average period of an inputted pulse signal from the sensor can be quickly calculated in a range from the small flow to the large flow.
On the other hand, in such a flow sensor, the measured flow rate is converted into a fixed signal form and outputted to an external apparatus by a flow transmitter. Generally, in such a flow transmitter, 8-bit digital data corresponding to the measured value is converted into the above fixed signal form and transmitted.
And, in such a flowmeter, it is known that a measured flow rate (i.e. an instantaneous flow rate) is integrated (namely, totalized) so as to obtain an integrated flow rate and the integrated flow rate in addition to the instantaneous flow rate are indicated at an indicating portion.
Further, as an application example of such a flowmeter, a flow detecting switch is proposed, wherein a flow rate of the fluid flowing through the flow-channel is compared with a reference flow value (a preset flow rate value) to be preset in advance, and a signal for controlling external equipment is outputted according to the comparison result. (Japanese Patent Application Laid-open NO. 9-89613)
In this proposed flow detecting switch, the period of a pulse signal having the frequency corresponding to the period of Karman vortexes is measured, and its flow rate value is obtained based on the measured period while referring to a table. And, periods of a plurality of pulses are measured and averaged in order to improve the measurement accuracy. And, a measured flow rate is compared with a preset threshold value, the comparison result is indicated by means of LED, and the base of a transistor whose collector is connected to an output terminal is applied with a voltage, thereby controlling the power supply to a load or the like connected to the output terminal.
Generally, in a frequency measuring method for counting the number of input pulse, the resolvability for low input-frequency should not be good regardless of the gate-time value. For example, when the gate-time is one second, the frequency not less than 200 Hz is required to obtain an accuracy of xc2x10.5%.
And, in a case of the period measuring, the measurement resolvability for high input-frequency is coarse, and therefore sufficient accuracy is not obtained regardless of the length of the period or a value of the reference clock for the measurement. Therefore, a count number not less than 200 is required for example.
And, in the above frequency measuring apparatus, since the timer to which a time not less than 2 times of the period of a measurement bottom frequency is set is used, when the frequency near the measurement bottom frequency is measured, a measuring time not less than 2 times of the period is required. And, a N-times period of a pulse signal is counted by the clock counter before the completion of the count timer of the timer, the detection and the count are stopped when the above count value has reached the specified value, and a frequency measured value is calculated from the above count value. In this case, because a measuring of the whole preset time of the timer has not been made, the true frequency of the inputted pulse signal is not measured.
And, according to the above average period calculation apparatus, though the average period of an inputted pulse signal from the sensor can be quickly calculated in a range from a small flow to a large flow by setting four or five parameters, the structure is complicated since the pulse number counting means, the pulse period measuring means, the first and second pulse period comparing means, and the first and second average period calculating means are used. And, in case that the processings by these means are executed by means of a microcomputer, enormous memory is required in the program, and a long time is required for the computing processing. And, since the average period calculation processing occupies a lot of time, the other processings would not often be difficult to be executed.
Further, as described above, the 8-bit digital data corresponding to the measured value is converted into the fixed signal form and transmit in the flow transmitter. In this case, since the error comes to about xc2x10.4% due to an digital quantization error of xc2x11 digit, the digital data with a lager bit-number than 8-bit should be used for preventing propagation of the error in order to output a high quality signal basing on a high accuracy detection toward an external apparatus.
And further, in the conventional flowmeter having a function to indicate an integrated flow rate value, since the integrated flow rate is indicated on the fixed-point system, there should be an inconvenience on reading a large integrated flow rate value.
Still further, as stated above, in the Karman vortex type flow sensor, since a flow measurement range and the frequency of the generated Karman vortexes are decided by a diameter of the flow-channel and material of the pipe portion of the flow sensor, it is preferably required that a single kind of flowmeter is easily applicable to the pipe portion in which the flowmeter is inserted.
And still further, in the above flow detecting switch, since the period of the Karman vortexes is measured, when the frequency is high, that is, the period is short, sufficient measurement resolvability is not obtained, that is, the measurement accuracy is not good. And, since the flow rate is decided from the frequency obtained from a reciprocal number of the period by referring to a data table, one flow rate data is defined correspondingly to one frequency, whereby the memory of enormous capacity is required. Therefore, since a data table for various diameters and the control program could not be stored in one microcomputer, a plurality of microcomputers should be prepared.
Further, a high-speed switch output is sometimes required for the flow detecting switch according to its use. For example, in a welding gun or an electric spark machining machine, when the above flow detecting switch is arranged on a flow-channel of cooling water so that an abnormal state of equipment can be detected based on a change of the flow rate of the cooling water, a high-speed switch output is required in order to prevent an overheat or the like of the equipment.
In view of the foregoing, an object of the present invention is to provide a flowmeter with simple processing, simple operation, low price, and high accuracy.
Further, another object of the present invention is to provide a flowmeter superior in utility by increasing an indication digit number of an integrated flow rate value.
Still further, another object of the present invention is to provide a flowmeter wherein the characteristics of a flow-channel can be easily selected with the operation of a key operation portion.
Still further, another object of the present invention is to provide a flow detecting switch which is easy to use since a setting mistake and an operation mistake can be prevented and the preset value can be easily checked and changed when used as a switch for transmitting an output signal to an external control unit after detecting an instantaneous flow rate and after comparing it with a preset value.
Still further, another object of the present invention is to provide an instantaneous flow rate detection switch which can carry out an switch output operation with a high speed when an accident happened to the flow of fluid.
Still further, another object of the present invention is to provide a flowmeter (an instantaneous flow rate transmitter) which can output a transmission signal at a fixed timing.
Still further, another object of the present invention is to provide a flowmeter (an instantaneous flow rate transmitter) which can output a high quality transmission signal to an external apparatus.
And, in the above conventional liquid mixing system, a local monitoring board is arranged neat the flowmeter or and the solenoid valve installed on a pipe, and a plurality of totalizers are arranged in the local monitoring board. Accordingly, since the local monitoring board is necessary other than the concentrated monitor, the system is not simple. And, since wires or cables are necessary between the concentrated monitors and the totalizers, a lot of man-hours and costs are required.
Therefore, though it would be possible to directly input the flow rate signal of the flowmeter in a sequencer forming the concentrated monitor without using the totalizers, a processing program for the sequencer to carry out a batch integrating processing and the required memory become large.
And, though the above totalizers 105A-105C integrate flow detection signals FA-FC inputted from the above flowmeters 104A-104C, compare the integrated results and preset integrated batch flow rate values, and output the above on/off control signals SC to the respective solenoid valves 103A-103C, there could often be a case that an integrated result has already exceeded the preset integrated batch flow rate value when the integrated result has been calculated.
Referring to FIG. 43, this situation will be described. The vertical axis indicates the integrated flow rate value (in cubic meter or little), the horizontal axis indicates the time, Qbs designates the above-described preset integrated batch flow rate, and Qb designates the integrated batch flow rate. As described above, since the totalizers 105A-105C calculate the integrated batch flow rate values Qb by integrating the flow detection signals FA-FC inputted from the flowmeters 104A 104C every measuring period, actual outputs of totalizers 105A-105C are outputted at time intervals corresponding to the above measuring period. In FIG. 43, Qbnxe2x88x921, Qbn, Qbn+1 designate the integrated batch flow rate values, respectively, at the respective times of tnxe2x88x921, tn, tn+1 having the above time interval therebetween. Accordingly, even if the integrated batch flow rate value Qb agrees with the preset integrated batch flow rate value Qbs at time t0, since it is judged that the integrated batch flow rate value Qb has actually reached the preset integrated batch flow rate value Qbs at each of the totalizers 105A-105C, the on/off control signal SC is outputted at time tn+1. In a fact, since time required for calculation of the flow rate at the above flowmeters 104A-104C and time required for the integration at the totalizers 105A-105C are necessary, a time delay of a maximum of about one measuring period arises.
Accordingly, since the above solenoid valves 103A-103C work in larger flow rate values than the preset integrated batch flow rate values Qbs, a good fluid mixture can not be obtaind.
In view of the foregoing, an object of the present invention is also to provide a flowmeter, a control method thereof, and a recording medium having recorded a control program, wherein downsizing of the system is attained, a wire or cable arrangement process can be omitted, and the load of a concentrated monitor can be reduced.
And, another object of the present invention is to provide a flowmeter, a control method thereof, and a recording medium having recorded a control program, wherein high speed and highly accurate measurement can be carried out with a simple structure.
Further, another object of the present invention is to provide a flowmeter, a control method thereof, and a recording medium having recorded a control program, wherein an automatic operation can be carried out, while outputting/inputting signals between external apparatuses such as a concentrated monitor and the like.
In order to achieve the above object, in the present invention, a flowmeter is characterized by comprising: a sensor portion to output a pulse signal having a frequency corresponding to a flow rate to be measured; a first signal generating portion to generate a first signal having a first period; a first measuring portion to count number of pulses outputted from the sensor portion during the first period according to the first signal; a second measuring portion to measure position of the first signal in a pulse including the first signal in its period; a first arithmetic portion to calculate the frequency of the pulse signal every first period on a basis of both of a count value by the first measuring portion and a measurement result by the second measuring portion; and a second arithmetic portion to calculate an instantaneous flow rate on a basis of the frequency calculated.
Further, the flowmeter is characterized by comprising a means for invalidating the measurement result when a state that a pulse period of the pulse signal is equal to a period corresponding to a commercial alternating current power supply frequency has occurred successively fixed times.
Further, the flowmeter is characterized in that the indicating portion indicates the integrated flow rate, while switching indication of upper figures and lower figures of the integrated flow rate in turn.
Further, a flow detecting switch of the present invention is characterized by comprising a sensor portion to output a signal corresponding to a measured flow rate, an instantaneous flow rate measuring portion to measure an instantaneous flow rate on a basis of the signal from the sensor portion, a comparing portion to compare the instantaneous flow rate value calculated by an arithmetic portion with a preset flow rate value having been preset, and a switch output portion to output a comparison result of the comparing portion as a digital signal, wherein the comparing portion has two operation modes of a normal mode to compare the instantaneous flow rate value calculated every first period with the preset flow rate value and a high-speed mode to compare the instantaneous flow rate value calculated every second period shorter than the first period with the preset flow rate value.
Further, the flowmeter is characterized in that the instantaneous flow rate measuring portion has a first signal generating portion to generate a first signal having a first period, a first measuring portion to count number of pulses included in a pulse signal outputted from the sensor portion during the first period according to the first signal, a second measuring portion to measure a position of the first signal in a pulse including the first signal in its period, and a first arithmetic portion to calculate a frequency of the pulse signal every first period on a basis of both of a count value by the first measuring portion and a measurement result by the second measuring portion.
Further, the control method of the flowmeter is characterized by further comprising a sixth step to compare the instantaneous flow rate calculated at the fourth step and a preset flow rate value having been preset and to output a digital signal corresponding to a comparison result to an external apparatus.
Further, a control method of a flow detecting switch for measuring an instantaneous flow rate value, for comparing the measured instantaneous flow rate value and a preset flow rate value having been preset, and for outputting a digital signal corresponding to a comparison result to an external apparatus is characterized by comprising the steps of: a first step to calculate the instantaneous flow rate value on a basis of a signal, corresponding to a measured flow rate, from a sensor portion; a second step to compare the preset flow rate value and the instantaneous flow rate value calculated at the first step; and a third step to output a digital signal corresponding to a comparison result of the second step, wherein the second step has two operation modes of a normal mode to compare the instantaneous flow rate value calculated at the first step and the preset flow rate value every first period and a high-speed mode to compare them every second period shorter than the first period.
Further, a flowmeter for carrying out flow measurement on a basis of a flow detection signal outputted from a flow sensor according to a flow rate of a measured fluid is characterized by comprising an instantaneous flow rate calculating means to calculate an instantaneous flow rate every fixed measuring period on a basis of the flow detection signal, an integrated flow rate calculating means to calculate an integrated flow rate by integrating the instantaneous flow rate, a judging means to judge whether or not the integrated flow rate has reached a preset integrated flow rate, an integration completion informing means to output an integration completion signal when the integrated flow rate has reached the preset integrated flow rate, an estimating operation means to calculate an estimated value of the integrated flow rate calculated by the integrated flow rate calculating means at a next measuring time-point and to judge whether or not the estimated value of the integrated flow rate reaches the preset integrated flow rate, and a measuring period controlling means to change the measuring period of the instantaneous flow rate calculating means to a shorter period when the estimating operation means has judged that the estimated value of the integrated flow rate at the next measuring time-point reaches the preset integrated flow rate.
Further, the flowmeter is characterized in that the integrated flow rate calculating means starts an integrating operation of the instantaneous flow rate according to an integration start direction signal supplied from an outside.
Further, the flowmeter is characterized in that the integrated flow rate is reset when the integration start direction signal has continued not less than a first fixed time and below a second fixed time, the integrating operation by the integrated flow rate calculating means is stopped when the integration start direction signal has continued not less than the second fixed time, and the integrating operation is restarted when an input of the integration start direction signal has disappeared.
Further, the flowmeter is characterized in that the estimating operation means calculates an estimated value of the integrated flow rate at the next measuring time-point by using the instantaneous flow rate having been calculated.
Further, the flowmeter is characterized in that the instantaneous flow rate calculating means has a first signal generating portion to generate a first signal having a first period, a first measuring portion to count number of pulses included in the flow detection signal inputted during the first period according to the first signal, a second measuring portion to measure a position of the first signal in a pulse including the first signal in its period, and a first arithmetic portion to calculate a frequency of the flow detection signal every first period on a basis of both of a count value by the first measuring portion and a measurement result by the second measuring portion, and a second arithmetic portion to calculate the instantaneous flow rate on a basis of the frequency having been calculated.
Further, the flowmeter is characterized by further comprising a manual operating portion for enabling the preset integrated flow rate to be changed.
Further, a control method of a flowmeter for carrying out flow measurement on a basis of a flow detection signal outputted from a flow sensor according to a flow rate of a measured fluid is characterized by comprising the steps of: an instantaneous flow rate calculating step to calculate an instantaneous flow rate every fixed measuring period on a basis of the flow detection signal; an integrated flow rate calculating step to calculate an integrated flow rate by integrating the instantaneous flow rate; a judging step to judge whether or not the integrated flow rate has reached a preset integrated flow rate; an integration completion informing step to inform of an integration completion when the integrated flow rate has reached the preset integrated flow rate; an estimating operation step to calculate an estimated value of the integrated flow rate calculated by the integrated flow rate calculating step at a next measuring time-point and to judge whether or not the estimated value of the integrated flow rate reaches the preset integrated flow rate; and a measuring period controlling step to change the measuring period of the instantaneous flow rate calculating step to a shorter period when the estimating operation step has judged that the estimated value of the integrated flow rate at the next measuring time-point reaches the preset integrated flow rate.
Further, the control method of the flowmeter is characterized in that the integrated flow rate calculating step starts an integrating operation of the instantaneous flow rate according to an integration start direction signal supplied from an outside.
Further, the control method of the flowmeter is characterized in that the integrated flow rate is reset when the integration start direction signal has continued not less than a first fixed time and below a second fixed time, the integrating operation by the integrated flow rate calculating step is stopped when the integration start direction signal has continued not less than the second fixed time, and the integrating operation is restarted when an input of the integration start direction signal has disappeared.
Further, the control method of the flowmeter is characterized in that the estimating operation step calculates an estimated value of the integrated flow rate at the next measuring time-point by using the instantaneous flow rate having been calculated.
Further, the control method of the flowmeter is characterized in that the instantaneous flow rate calculating step comprises the steps of: a first step to count number of pulses included in the flow detection signal inputted during a first period according to a first signal having the first period, a second step to measure position of the first signal in a pulse including the first signal in its period, a third step to calculate a frequency of the flow detection signal every first period on a basis of both of a count value by the first step and a measurement result by the second step, and a fourth step to calculate the instantaneous flow rate on a basis of the frequency calculated at the third step.
Further, in the present invention provided is a recording medium having recorded a control program for making a flowmeter carry out flow measurement on a basis of a flow detection signal outputted from a flow sensor according to a flow rate of a measured fluid, which recording medium having recorded a control program makes a flowmeter: calculate an instantaneous flow rate every fixed measuring period on a basis of the flow detection signal; calculate an integrated flow rate by integrating the instantaneous flow rate on a basis of an instruction from an outside; judge whether or not the integrated flow rate has reached a preset integrated flow rate; inform of an integration completion when the integrated flow rate has reached the preset integrated flow rate; calculate an estimated value of the integrated flow rate calculated at a next measuring time-point and judge whether or not the estimated value reaches the preset integrated flow rate; and change the measuring period for calculating the instantaneous flow rate to a shorter period when it has been judged that the estimated value of the integrated flow rate reaches the preset integrated flow rate.